1. Field of the Invention
The present invention relates to an improvement of a pulse generating circuit for generating steep pulse output by rapidly discharging a charge of a capacitor.
2. Description of Prior Art:
In order to generate spark for firing a flash bulb for photography, a gas lighter or a stove, pulse generating circuits having relatively simple structure which discharge charge of a capacitor to generate steep pulse output by the discharge current have been used.
FIG. 1 is a circuit diagram of one embodiment of the conventional pulse generating circuit.
In FIG. 1, the reference numeral (1) designates a DC power source such as a dry battery (9 to 12 volts); (2) designates a starting switch; (3) designates a resistor for charge; (4) designates a capacitor; (5) designates a trigger element; (6) designates a thyristor; (7) designates a load and (8) and (9) designate output terminals.
The operation of the circuit will be described.
When the starting switch (2) is turned on, the capacitor (4) is charged by the DC power source (1) through the switch (2) and the resistor (3) for charge. The time constant for charge is given by the resistance of the resistor (3) and the capacitance of the capacitor (4). When the charged voltage of the capacitor (4) reaches to the switching voltage (E.sub.s) of the trigger element (5), the trigger element (5) is turned on to feed the trigger signal to the gate of the thyristor (6) to turn on the thyristor. As the result, the charge of the capacitor (4) is discharged through the load resistor (7) and the thyristor (6) whereby the pulse output is generated between the output terminals (8), (9) of both ends of the load resistor (7).
FIG. 2 shows the voltage waveform of the capacitor (4) in such operation mode. The charging operation is initiated at the time of turning on the switch (2).
When the charged voltage reaches the switching voltage (E.sub.s) of the trigger element at the point (t.sub.1), the charge is immediately discharged to be zero, and the thyristor (6) is immediately turned off after the discharge. Accordingly, the charging operation of the capacitor (4) initiates again. The operation is repeated.
Accordingly, as shown in FIG. 2, the charge and discharge are repeated in a cycle of period t.sub.1 given by the specific switching voltage (E.sub.s) in ON state of the switch (2) whereby the pulse output is continuously obtained between the output terminals (8), (9).
However, in the circuit, the initiation of discharge is decided by the specific switching voltage (E.sub.s) given by the trigger element (5). If the voltage of the power source (1) decreases to be incapable of charging to the specific switching voltage (E.sub.s), the discharge is not caused and the pulse output cannot be obtained. This is the fatal disadvantage.
Accordingly, when the voltage of the DC power source (1) is highly varied, the switching voltage (E.sub.s) of the trigger element (5) should be set in relatively low voltage under the consideration of the variation. Accordingly, the voltage of the pulse output is decreased to be difficult to obtain the output voltage being proportional to the voltage of the DC power source (1). Moreover, if the voltage of the DC power source (1) is decreased below the switching voltage (E.sub.s), the output is not given. It is difficult to discriminate the phenomenon from the failure of the circuit and to find the cause of the non-output.